The present invention relates to the production of magnesium metal by the reduction of magnesium oxide at elevated temperatures in the presence of an aluminum-silicon alloy reducing agent and a molten oxidic slag in an electric furnace, and the condensation of vaporized magnesium in a condenser.
An advantageous method of producing magnesium lies in the chemical reduction of magnesium oxide with a reducing agent in the presence of a molten oxide slag in an electric furnace. A variety of metallothermic processes, as they are commonly called, for the production of magnesium employing various reducing agents, various types of reactants, and under various conditions of temperatures and pressures have been proposed.
In general, the various metallothermic processes are concerned with the production of metallic magnesium by the reduction of magnesium oxide with a metal reductant at elevated temperatures. Magnesium oxide, usually in the form of calcined dolmite (dolime) or calcined magnesite or mixtures thereof, is caused to react with a metallic reducing agent, such as silicon, aluminum, calcium or mixtures or alloys thereof, in the presence of a molten slag bath in a furnace at temperatures in excess of 1300.degree. C., to release magnesium vapor which may be condensed and collected. Some of the processes are carried out in the presence of an inert gas.
An early process of this type, called the Pidgeon process provides for the production of magnesium by the reduction of magnesium oxide with ferrosilicon.
A more recent process is exemplified by U.S. Pat. No. 2,971,833 and is known as the Magnetherm process. The process is operated under a very high vacuum (pressure ranging between 5 and 20 millimeters of mercury) and at a temperature of about 1500.degree. C. utilizing an electric furnace. Silicon is employed as the reductant, preferably in the form of ferrosilicon containing 70- 80 percent Si, silicon of a purity in excess of 97 percent, or an aluminum-ferrosilicon. Care is taken to avoid that the silicon content of the residual ferrosilicon drops below 33.5 percent. A magnesia-containing substance is dissolved in a liquid slag consisting essentially of lime, silica and alumina wherein the ratio of calcium oxide to silicon dioxide and the ratio of aluminum oxide to silicon dioxide are controlled.
Processes employing aluminum-silicon alloy as the reductant are exemplified by U.S. Pat. Nos. 3,427,152, 3,441,402, 3,579,326, 3,698,888, 3,761,247, 3,761,248. In the first patent, magnesium is produced by thermal treatment of a mixture of magnesium oxide and a reducing agent wherein the necessary heat is imparted by pre-heated inert gas which is counter-currently flowed with respect to the magnesium oxide. The second patent relates to a process for the continuous production of magnesium metal which comprises introducing a mixture of magnesium ores into a submerged arc electric furnace together with a reducing agent. Furnace temperature is maintained below about 1500.degree. C. and operated at one atmosphere. The third patent provides for a process for producing magnesium by reducing magnesium oxide from an oxidant containing a major proportion of magnesia with a metallic aluminum-silicon alloy reductant having a ratio of silicon to aluminum of at least 0.8 to 1.0 at a temperature of at least 1400.degree. C. and in the presence of a molten slag comprising 5- 25 percent magnesium oxide and less than 30 percent calcium oxide. The fourth patent relates to the production of magnesium by the metallothermic reduction of magnesium oxide in a reaction zone, by means of a metallic reducing agent, and in a reaction-condensation system containing in its vapor space an inert gas at a partial pressure of 0.25- 2 atmospheres. The latter two patents relate to similar processes, but in one the reaction is carried out in the presence of hydrogen at a pressure of at least about one atmosphere and in the other, the production of magnesium is induced in part by the passage of inert gas through the reaction-condensation zone.
Other metallothermic processes for producing magnesia disclosing an aluminum-silicon alloy as a reductant, as well as other reductants, are disclosed in U.S. Pat. Nos. 1,311,378, 2,099,151, 2,126,825, 2,351,488, 2,351,489, 2,527,722 and 2,527,724.
U.S. Pat. No. 3,658,509 also discloses the use of aluminum-silicon alloy as the reductant as well as aluminum and silicon. In such process, an inert gas is used to obviate at least in part the need of a high vacuum. U.S. Pat. No. 3,681,053 also discloses the use of an aluminum-silicon alloy as the reductant for the production of magnesium from magnesium oxide at elevated temperatures in the presence of molten oxidic slag, but requires that the alloy contain at least 80 percent silicon and no more than 20 percent aluminum and minor amounts of iron or other non-reactive components.
U.S. Pat. Nos. 3,475,162 and 3,567,431 disclose metallothermic processes for producing magnesium, wherein the reducing agent is an aluminum-silicon-iron alloy and an aluminum-silicon-iron-titanium alloy, respectively.
The primary object of this invention is to provide a process for production of magnesium metal wherein aluminum-silicon alloy is reacted with a calcium magnesium aluminum silicate slag, or with magnesium oxide in the presence of said slag, which eliminates the disadvantages of the prior art processes, while retaining the benefits thereof.
Another object of this invention is to provide a process for production of magnesium metal at elevated temperatures and reduced pressures by reaction of an aluminum-silicon alloy with a calcium magnesium aluminum silicate slag, or with a magnesium oxide in the presence of said slag.
An important object of the present invention is to provide a process of the foregoing type which is economical.
Other objects and advantages of the present invention will become more readily apparent from a consideration of the description hereinafter.